Naphthalene, a commonly used industrial solvent, causes pneumotoxicity in mice but not in rats. The mechanism appears to depend on the enzymatic activation of naphthalene by cytochrome P-450 present in mouse lung. In the past we have isolated a cytochrome P-450 from mouse liver designated P-45Om5Ob that metabolizes naphthalene almost solely to only on epoxide, namely (lR,2S)-naphthalene 1,2-oxide. In order to characterize this protein, polyclonal anti-P-45Om5Ob antibodies were raised in rabbit and used to screen cDNA libraries. Ten positive clones were isolated. A cDNA from mouse lung cDNA library containing a complete protein reading frame was sequenced. This clone was expressed in both COS cells and yeast. The expressed enzyme displayed the naphthalene hydroxylase activities. The metabolic profiles of naphthalene conjugates and hydroxybiphenyl expressed in this clone transformed yeast microsomes confirmed unequivocally that this clone is naphthalene hydroxylase.